Characteristics and controlling factors of lacustrine tight oil reservoirs of the Triassic Yanchang Formation Chang 7 in the Ordos Basin, China

Abstract

Tight sandstone oil reservoirs have received increasing attention in petroleum exploration and exploitation. Previous research concerning tight oil reservoirs has predominantly focused on marine basins. However, in China, tight oil reservoirs are mostly distributed in lacustrine basins. Taking the tight oil sandstones of the Yanchang Formation Chang 7 oil reservoir interval (the Chang 7 for short) in the Ordos Basin as an example, five factors are identified that control their formation and occurrence. Firstly, three episodes of oil charging took place during the Early Cretaceous, which were 142–139 Ma, 128–122 Ma and 114–106 Ma, respectively. Continuous generation and episodic expulsion led to multiple episodes of oil accumulation. Secondly, widely distributed source rocks (TOC > 1.0%) provide hydrocarbons to the Chang 7. The distribution of tight oil was controlled by the outer boundary of the source rock distribution, while the transition areas between generation (expulsion) centers were the accumulation and enrichment zones in the Chang 7. Thirdly, with the favorable conditions for retained hydrocarbons, massive continuous tight reservoir beds (average porosity = 7.6% and average permeability = 0.15 mD) developed in the Chang 7 capable of forming large-scale successive tight oil reservoirs. Fourthly, different source-reservoir assemblages have different expulsion patterns and amounts, which led to different charging degrees and sizes of tight oil accumulations. Type III (source-reservoir interbeds) will be the most favorable target for exploration and exploitation in the Chang 7, Ordos Basin. Finally, under the driving mechanism of non-buoyancy migration and accumulation, pressure differences between source and reservoir rocks (PDBSR) serve as the primary expulsion and migration force for crude oil during the main accumulation periods in the Chang 7.